EMI (electromagnetic interference) is the generation of undesired radiated energy which has the potential for interfering with the operation of electronic systems. Regulatory bodies exist in most countries to set standards and guidelines for EMI, and these bodies also monitor emissions by industrial, commercial, and domestic systems to assure compliance. One method to assure compliance with regulatory requirements is to modulate a signal that is generating unacceptable EMI and tailor the energy power spectrum of the signal to distribute the radiated components and limit their amplitude. For example, EMI can be reduced by distributing the emitted energy of a clock source (periodic signal) over a wider frequency band by modulation techniques known in the prior art. This technique is also termed spread spectrum EMI reduction.
In the case of frequency modulation, the modulation profile describes the instantaneous frequency of the modulated signal at any instant. In more general terms, it gives an indication of how much the frequency deviates from its average value (bandwidth), how fast this deviation takes place (the rate of modulation), and in what manner (sinusoidal, triangular, parabolic) the variation occurs. All of these parameters have great influence on the EMI emissions of a system.
Due to the numerous modulation profiles which can be generated, and the varieties of physical configurations that may be used, it is extremely unlikely that two spread spectrum noise reduction systems can be interfaced without encountering problems related to the superposition of these modulating signals on one another.
However, systems that employ separate spread spectrum EMI reduction techniques are often combined into a single device. For example, in laptop (notebook) computers today, there are two distinct modules that re required to obtain EMI certification from the associated regulatory body (FCC, CISPER, etc). They are
1) The motherboard and
2) The LCD panel module.
Manufacturers of these devices (notebook computers and similar appliances) typically connect LCD panels from different panel manufacturers to motherboards from other sources. In order to achieve EMI compliance, a large number of notebook computer motherboard designers and manufacturers use Spread Spectrum clocking techniques. In particular, these techniques see wide application in the transfer of “graphics data” from the graphics controller on the motherboard, to the LCD panel, over a cabled connection or trace lines. This methodology has been in use for more than 5 years.
The display panel manufacturer must also meet EMI compliance guidelines set by the same regulatory bodies (FCC, CISPEWR etc) and are now exploring the use of these very same Spread Spectrum clocking techniques to control emissions from data on the panel. When these two modules (the motherboard and panel), each using their separate Spread Spectrum clocking techniques, are linked to each other to create the final product, a complete notebook computer, the spread spectrum clocking modulations from the motherboard will be injected into the panel module and the panel will superimpose its own spread spectrum modulation on top of the incoming signal. This will lead to “secondary modulation” effects and errors in the data transfer. This may result in “screen flickering or scrolling” and other subtler visual degradation, or other, more severe problems.